This invention relates to the manufacture of paper and, more particularly, to the bleaching of mechanically-produced pulps containing lignin, including both softwood and hardwood, so as to reduce yellowness and improve brightness.
Mechanically-produced pulps, softwood and thermomechanical pulps containing lignin, as opposed to chemically produced wood pulps, used for the production of paper, have traditionally been bleached to improve the whiteness thereof. Newsprint, which contains a relatively high lignin content, is either not bleached or only mildly bleached, with the result that it is usually of a darker quality than paper produced from fully bleached pulp, and tends to darken further when exposed to light, a phenomenon known as xe2x80x9creversionxe2x80x9d.
Whiteness or xe2x80x9cbrightnessxe2x80x9d of paper is conventionally measured by brightness measurements based on the reflectance of light at a wavelength of 457 nm, using such instruments as an Elrepho brightness meter. There is, however, another measure of whiteness which is even more significant and that is the degree of yellowness (CIE yellow color coordinate b*). Bleached softwood pulps usually have a brightness of about 70-90% Elrepho and a yellowness b* of 8 or more (often 9-10 at a brightness of 76-78 points ISO), and there are several known methods for achieving these results, using either an oxidative process using hydrogen peroxide under strongly basic conditions or reductive processes using hydrosulfite (dithionite) or combinations thereof.
Currently, hydrogen peroxide bleaching of pulps is generally performed at an alkaline pH of xcx9c10-11 in order to facilitate the ionization of hydrogen peroxide to HOOxe2x88x92. This is accomplished using a mixture of a water soluble alkali (usually sodium hydroxide) and a buffer (usually sodium silicate). This allows maintenance of the pH at a level that permits effective bleaching, and also buffers the solution to minimize peroxide decomposition, which increases with increasing alkalinity. Often a small amount of magnesium ion is added to form a colloidal suspension of magnesium silicate, which may help, by adsorption, to inactivate metallic catalysts of peroxide decomposition such as Mn and Fe ions.
Silicates, however, can result in buildup of silica scale, especially when bleach process water is recycled. The alkali (NaOH) is expensive and must be neutralized (soured) and/or washed out after the bleaching process. In addition, alkali is known to have a darkening effect on pulps. Alkali-induced darkening reactions in the pulp compete with bleaching and effectively limit the brightness to ISO xcx9c84. Alkali-induced depolymerization and xe2x80x9cpeelingxe2x80x9d reactions of cellulose are detrimental to fiber length and strength. Rates of the various alkali-induced reactions increase at higher temperatures and limit practical bleaching temperatures to xcx9c80xc2x0 C.
There is an ongoing need for improved but inexpensive mechanical and thermomechanical pulps having improved brightness of the order of 85 and decreased b* values of the order of less than 5; and with greater stability of the optical properties, i.e., decreased reversion.
An object of the present invention is to provide improved mechanical and thermomechanical pulps having a brightness of 80 (ISO) or more and a yellowness (b*) value 6 or lower. A further object is to provide an improved process for producing such pulps, wherein added alkali and/or silicate is not employed but equivalent optical characteristics to processes employing alkali, optionally with silicate, are obtained. A still further object is to provide means to break the xe2x80x9cbrightness ceilingxe2x80x9d of xcx9c84-85 ISO common in alkali-based processes.
By one aspect of this invention, a method for bleaching an unbleached softwood or hardwood pulp is provided, using hydrogen peroxide without added alkali for activation. The method comprises treating (digesting) said pulp in an aqueous hydrogen peroxide solution containing up to about 10% carbonate at a temperature in the range of 15 to 80xc2x0 C., and preferably 15-60xc2x0 C. The solution may be up to about 30% by weight of pulp. The carbonate is preferably magnesium or calcium carbonate. The pH range of the method is preferably about 6 to about 7.
By a second aspect of the invention, a method for bleaching an unbleached softwood or hardwood pulp is provided, comprising treating (digesting) said mechanical pulp in an aqueous hydrogen peroxide solution containing at least 4% H2O2 and at least 6% alkaline earth metal carbonate by weight of pulp at a temperature of at least 15xc2x0 C., more preferably at least 40xc2x0 C., still more preferably at least 60xc2x0 C. Still more preferably, the temperature range is 75xc2x0 to 120xc2x0 C., more preferably 80xc2x0 to 115xc2x0 C., still more preferably, 80xc2x0 to 110xc2x0 C. At such temperatures, the reaction may proceed for about one to three hours, with shorter times required at higher temperatures. No alkali (NaOH) is added to the reaction mixture. The alkaline earth metal carbonate is optionally calcium carbonate and preferably magnesium carbonate. Preferably the alkaline earth metal carbonate is at least 10% by weight of pulp, more preferably at least 20%, at least 25%, at least 30%, at least 35% or at least 40%. Preferably, the H2O2 is at least 5%. But, the H2O2 may be at least 10%, at least 15% or at least 20%. The invention provides the advantage of employing temperatures greater than the practical xe2x80x9cceilingxe2x80x9d of xcx9c80xc2x0 C. for alkali bleaching reactions, without alkali induced darkening of the pulp.
According to this aspect of the invention, a b* value of less than 6 is preferably obtained. An ISO brightness value of at least 85 is preferably obtained, still more preferably an ISO brightness value of at least 90 obtained. In a particularly preferred embodiment, the wood pulp is a mechanical hardwood pulp, the ISO brightness value obtained is in the range of 90-96 and the b* value is less than 2.6.
If desired, the bleaching method of the present invention can be coupled with a formaldehyde/carbonate process as described in our U.S. application Ser. No. 08/936,200 (1997). In said process, a wood pulp containing lignin, such as a bleached hardwood or softwood pulp, is digested in an aqueous formaldehyde solution containing carbonate, preferably an alkali or alkaline earth metal carbonate, and preferably at ambient temperature. Yellowness b* is preferably reduced to less than 8, more preferably to less than 6, still more preferably to less than 5. This coupled method of a peroxide/carbonate treatment and a formaldehyde/carbonate treatment is also encompassed by the invention.
Preferably, the invention provides improved b* without significant delignification. Delignification commonly occurs under strongly acidic or strongly basic conditions and can result in significantly reduced yields.
The invention provides several advantages over conventional hydrogen peroxide bleaching:
1. Lower cost. An alkaline earth metal carbonate such as magnesium carbonate is very inexpensive as compared to sodium hydroxide and sodium silicate.
2. Increased brightness values and reduced b* values for both softwood and hardwood mechanical pulps over conventional bleaching. This increases the number and variety of applications for the product, significantly expanding its marketability.
3. Much less light sensitive pulps. This is indicated by their lower b* values, limiting light induced reversion processes.
4. Avoidance of problems with silica scale and alkaline corrosiveness on equipment.
5. Environmental benefits due to the avoidance of the use of soluble alkali, silicate and acid souring agents.
The inventors have found the inclusion of carbonate, particularly alkaline earth metal carbonate, in a hydrogen peroxide bleaching process to be beneficial and effective without need for added alkali (sodium hydroxide). The method of the invention is carried out at approximately neutral pH, and less than about pH 8, thus avoiding the disadvantages and drawbacks of commonly used alkali bleaching processes.
Although it is known to use sodium carbonate at fairly high concentrations in some bleaching reactions, the use of substantial levels of alkaline earth metal carbonates, e.g., calcium carbonate or magnesium carbonate, has heretofore been unknown. Previously, low levels (xe2x89xa61%) of magnesium salts, such as, e.g., magnesium silicate, have been employed in bleaching reactions optionally containing sodium carbonate, to adsorb and inactivate metallic catalysts of peroxide decomposition such as Mn and Fe ions. However, prior to the present invention, the advantages of employing levels of magnesium carbonate of 10-fold or greater magnitude have been unrecognized.
According to the invention, a method for bleaching an unbleached softwood or hardwood pulp is provided, comprising digesting said mechanical pulp in an aqueous hydrogen peroxide solution containing at least 4% H2O2 and at least 6% alkaline earth metal carbonate by weight of pulp at a temperature of at least 60xc2x0 C. No alkali (NaOH) is added to the reaction mixture. The alkaline earth metal carbonate is optionally calcium carbonate and preferably magnesium carbonate. Preferably the alkaline earth metal carbonate is at least 10% by weight of pulp, more preferably at least 20%. Preferably, the H2O2 is 5-15%. The temperature range is preferably 75xc2x0 to 120xc2x0 C., more preferably 80xc2x0 to 115xc2x0 C., still more preferably, 80xc2x0 to 110xc2x0 C. At such temperatures, the reaction may proceed for about one to three hours, with shorter times required at higher temperatures.
Because of their low level of solubility in water (compared to that of e.g., sodium carbonate) an alkaline earth metal carbonate employed according to the method of the invention generally does not completely dissolve. Thus, bleaching mixtures containing, for example 10% magnesium carbonate, appear as a white suspension. Hardwood mechanical pulps have been found to be bleached by this new heterogeneous process to ISO brightness values of about 90 and b* values from  less than 5 to as low as xcx9c2. Softwood mechanical pulps have been found to reach brightness of about 85-87 and b* values of about 5-8.
Although the mechanisms by which this novel hydrogen peroxide bleaching process accomplishes the significant enhancements in b* and brightness values are incompletely understood, and without wishing to be bound by this explanation, it is believed that the process is initiated due to heterogeneous activation of the hydrogen peroxide by the insoluble alkaline earth carbonate. For example:
MgCO3+H2O2⇄Mg2++HCO3xe2x88x92+HOOxe2x88x92
In contrast to conventional processes it is not necessary to have any soluble alkali (OHxe2x88x92) to ionize the peroxide, and the process takes place at neutral or mildly basic pH (6-8). As a result bleaching can be accomplished at temperatures ranging from about 15xc2x0 C. to about 120xc2x0 C. No alkali induced darkening reactions compete with bleaching with the result that the xe2x80x9cbrightness ceilingxe2x80x9d of 84-85 which limits the conventional processes can be broken. Preferably, no silicate, e.g., sodium silicate is used.
Any alkaline earth metal may be employed according to the invention, but magnesium has the added beneficial effect of replacing manganese in the pulp, allowing its easier removal by chelation. As noted above manganese and iron ions are the the main cations responsible for decreasing the activity of hydrogen peroxide solutions in pulp bleaching.
According to one embodiment of the invention, using a 6% charge of both hydrogen peroxide and alkaline earth metal carbonate, an aqueous suspension of pulp (about 10-20% consistency), and a reaction time of 1 h at 50 to 60xc2x0 C., effects good pulp bleaching for both unbleached hardwood and softwood mechanical pulps. Values of brightness and b* at least as good, and in some cases better, than those obtained by conventional alkali peroxide bleaching have been obtained. The pH of the mixture remains about 6-7 throughout the treatment.
The significant drop in b* values renders the pulps visibly much whiter in appearance and they are accordingly believed to be less susceptible to light induced reversion processes. This is tested by accelerated photoreversion experiments done in a photoreactor under several 350 nm ultraviolet lamps. Papers made from pulps treated according to the invention are expected to photoyellow more slowly than papers made from control pulps, especially during initial irradiation.
The inventors believe that a further decrease in the reversion rate can be obtained if the pulps treated according to the invention are further treated by a reversion inhibitor, such as, for example, the polyethylene glycol bisthiols described in our U.S. application Ser. No. 08/261,275 (1996) and in our paper (Wan et al., xe2x80x9cSome mechanistic insights in the behavior of thiol containing antioxidant polymers in lignin oxidation processesxe2x80x9d, Res. Chem. Inter. 22: 241-253 (1996)), the disclosures of which are incorporated herein by reference. Suitable reversion inhibitors include polyethylene glycol bisthiolactate and polyethylene glycol bisthioglycolate.
The following Examples further illustrate the present invention and are not intended to be limiting in any respect. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, numerous equivalents to the specific procedures described herein. Such equivalents are considered to be within the scope of this invention and are covered by the claims.